It is expensive because the plant/local loop is very expensive to maintain, the switches and transport systems are very expensive to maintain.
The phone companies have hundreds of central offices, with millions of dollars in equipment in and power plant in them. They have realized efficiencies over the years and POTs is relatively cheaper than it was 30 years ago and long distance cost almost nothing.
With the advent of fiber through the 80's and 90's Bell System/RBOC's replaced all of the national long distance system and their inter office transport system. Upgraded all the central offices to near 100% digital and extend alot of fiber to the node in the local loop. Most of this that has been replaced over the past 30 years at the cost of 10 of billions is near obsolete and will be replaced. With faster transport and soft switch routing but it will be many years before it replaces pots.
I find it amazing that instead of lowering the price of something alot of people use they raise the price how many times in a year very stealth like. But they screw themselves in the end.
